The present invention relates to means for determining the electrical resistivity characteristics of compliant materials such as unvulcanized rubber-carbon black compounds.
It is well known that the resistivity of a vulcanized rubber-carbon black compound is dependent on the degree of mixing of the black and rubber in the mixing machine during preparation of the unvulcanized compound. During compounding of the unvulcanized rubber compound, the electrical resistivity of the compound tends to reach a minimum value when the black is just incorporated and in its minimum state of dispersion in the matrix. At this stage the carbon black forms a random, loose, and not completely coherent network. Past this point of minimum dispersion, however, the carbon black network is broken up into smaller domains which are more distantly spaced as more shear and mixing energy is dissipated into the compound. As a consequence of this destruction and dispersion of the carbon black network, the electrical resistivity of the compound increases. Therefore, electrical resistivity is a property which reflects the degree of dispersion of the carbon black (or other electrically conductive filler) and it is understandable that attempts should be made to use electrical resistivity measurement of a compliant material containing an electrically conductive filler as a means of monitoring the quality of dispersion of said filler. Where carbon black is concerned, high quality dispersions thereof in rubbers are generally desirable in order to assure development of optimum physical properties in the final vulcanizates. However, the time involved in the preparation of vulcanized form-stable samples mitigates against use of an electrical resistance test method as a rapid way of assessing the degree of dispersion of an electrically conductive filler in a rubber compound.
It can be assumed that a similar relationship as between degree of dispersion of an electrically conductive filler and resistivity of the ultimate vulcanizates also exists in the unvulcanized compound, in other words, prior to curing thereof. Here, however, the difficulties encountered in measuring unvulcanized compound resistivity are extensive and, insofar as is known to the present applicant, the literature does not mention systematic measurement of unvulcanized compound resistivity as a variable test for the quality of filler dispersion. The difficulties attendant resistivity testing of unvulcanized rubber compounds are outlined below.
Firstly, the instrument probe electrodes must make good contact with the compound under test and this implies that the electrodes must, of necessity, be forced onto the compound under high pressure. On the other hand, application of electrodes under high pressure to an unvulcanized rubber compound sample causes flow and distortion of the sample, thus interfering with the dimensional stability of the sample and upsetting the equilibrium state of the filler dispersion therein. This phenomenon results in erratic electrical resistivity test values. Heretofore, in order to give an unvulcanized rubber sample sufficiently stable and defined dimensions as to ameliorate the problem, it has generally been necessary to form the sample by pressing it for some time in a mold and at a somewhat elevated temperature. This expedient, however, also defeats the achievement of the goal of rapid measurement of unvulcanized compound resistivity. In accordance with the present invention, this problem has been resolved.
As used herein, the term "resistivity" refers to the apparent resistivity of a compound. Said apparent resistivity may also include a contribution from the contact resistance between electrodes and compound.